Aldolase dihydroxyacetone phosphate

Further steps m glycolysis use the d glyceraldehyde 3 phosphate formed m the aldolase catalyzed cleavage reaction as a substrate Its coproduct dihydroxyacetone phosphate is not wasted however The enzyme triose phosphate isomerase converts dihydroxyacetone phosphate to d glyceraldehyde 3 phosphate which enters the glycol ysis pathway for further transformations... 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

Table t. Products from Complementary Aldolase Catalyzed Additions of Dihydroxyacetone Phosphate to Simple Aldehydes... 

Figure 10.13 Aldol reactions catalyzed in vivo by the four stereocomplementary dihydroxyacetone phosphate-dependent aldolases.

Figure 10.18 Enzymatic in situ generation of dihydroxyacetone phosphate from fructose 1,6-bisphosphate (b), with extension to an in vitro artificial metabolism for its preparation from inexpensive sugars alongthe glycolysis cascade (a), and utilization for subsequent stereoselective carbon-carbon bond formation using an aldolase with distinct stereoselectivity (c).

Figure 10.19 Oxidative enzymatic generation of dihydroxyacetone phosphate in situ for stereoselective aldol reactions using DHAP aldolases (a), and extension by pH-controlled, integrated precursor preparation and product liberation (b).

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

Dihydroxyacetone phosphate (82) is a substrate for a-glycero-phosphate dehydrogenase, aldolase, and triose phosphate isomerase, and its O-alkyl ethers are intermediates in the biosynthesis of phospholipids. In neutral aqueous solution at 20 °C, dihydroxyacetone phosphate exists as an equilibrium mixture of the keto (82), gem-d o (83), and enol (84) forms, as shown by n.m.r. spectroscopy. The proportion of (82) to (83)... 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

The hexose phosphate, fructose-1,6-diphosphate, is split by aldolase into two triose phosphates glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Aldolase consists of four 40-kDa subunits. Three tissue-specific forms exist in human tissues aldolase A (ubiquitous and very active in the muscle), aldolase B (liver, kidney, and small intestine), and aldolase C (specific to the brain). These three isozymes have nearly the same molecular size but differ in substrate specificity,... 

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

Dihydroxyacetone phosphate reacts with D-glycerose in the presence of aldolases of muscle and liver to give D-fructose 1-phosphate (XII) exclusively, whilst DL-glycerose forms equimolar proportions of D-fructose 1-phosphate (XII) and L-sorbose 1-phosphate (XIII).65 Specificity of the enzyme is interesting in the light of Fischer and Baer s observations66 in... 

Jack-bean aldolase and liver aldolase catalyze the conversion of one mole of D-fructose 1-phosphate into one mole each of dihydroxyacetone phosphate and D-glycerose, and the reaction is reversible.73-77... 

L-Galactose is probably not produced from L-glycerose, since the latter inhibits glycolysis and, even so, reaction with dihydroxyacetone phosphate in the presence of aldolase yields L-sorbose 1-phosphate which, on stereochemical grounds, is also an unlikely precursor. A more plausible route is direct conversion from D-galactose (XXI) by complete reversal of stereo-... 

Recently, a chemoenzimatic catalized Henry reaction has been reported by El Blidi et al.53 Nitroaldol cyclization between the masked 3-hydroxy-4-nitrobutyraldehyde 72 and dihydroxyacetone phosphate (DHAP) 73, catalyzed by fructose-1,6-biphosphate aldolase (RAMA), afforded the nitro-cyclohexane 74 (Scheme 24). 

Aldolase 4.1.2.3 Fructose-1,6-diphosphate Dihydroxyacetone phosphate Formation of a hydrazone... 

Espelt, L., Parella, T., Bujons, J., Solans, C., Joglar, J., Delgado, A. and, Clapes, P., Stereoselective aldol additions catalyzed hy dihydroxyacetone phosphate-dependent aldolases in emulsion systems preparation and structural characterization of linear and cyclic iminopolyols from aminoaldehydes. Chem. Eur. J., 2003, 9, 4887. 

Figure 11.2 Pathway for conversion of fructose to acetyl-CoA. The enzyme fructokinase phosphorylates fructose to form fructose 1-phosphate. (The enzyme is present only in the liver.) Fructose 1-phosphate is cleaved by aldolase to form glyceraldehyde and dihydroxyacetone phosphate. Glyceraldehyde is phos-phorylated to form glyceraldehyde 3-phosphate, catalysed by the enzyme triokinase. Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate, catalysed by the isomerase. Glyceraldehyde 3-phosphate is converted to pyruvate by the glycolytic reactions (Chapter 6).

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

In Box 10.4 we saw that an aldol-like reaction could be used to rationalize the biochemical conversion of dihydroxyacetone phosphate (nucleophile) and glyceraldehyde 3-phosphate (electrophile) into fructose 1,6-diphosphate by the enzyme aldolase during carbohydrate biosynthesis. The reverse reaction, used in the glycolytic pathway for carbohydrate metabolism, was formulated as a reverse aldol reaction. 

Enzymes are very sophisticated systems that apply sound chemical principles. The side-chains of various amino acids are used to supply the necessary bases and acids to help catalyse the reaction (see Section 13.4). Thus, the enzyme aldolase binds the dihydroxyacetone phosphate substrate by reacting the ketone group with an amine, part of a lysine amino acid residue. This forms an imine that becomes protonated under normal physiological conditions. 

Aldolase catalyses both aldol and reverse aldol reactions according to an organism s needs. In glycolysis, the substrate fmctose 1,6-diphosphate is cleaved by a reverse aldol reaction to provide one molecule of glyceraldehyde 3-phosphate and one molecule of dihydroxyacetone phosphate. In carbohydrate synthesis, these two compounds can be coupled in an aldol reaction to produce fmctose 1,6-diphosphate. 

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